Light sensitive element



Filed Nov. e, 192e INVENTOR imr K. ZworyKin VI-ad f r'ATTORNEY I 20 drawings in which light sensitive cell em odying the principles Patented May 26, `1931 UNITED STATES PATENT oFFlcE .VLADIMIB K. ZWOBYKIN, 0F SWISSVALE, PENNSYLVANIA, ASSIGNOR TO WESTIIIG- HOUSE ELECTRIC & MANUFACTURING COMPANY, ACORPORATION OF PENNSYL- VANIL Ireirr SENSITIVE .ELEMENT Application led November l8, 1928. Serial Nox 317,598.

My invention relates to light sensitive electrieal conductors .and particularly to selenium cells.

One object of my invention is to produce 5j a light sensitive electrical conductor in which the fractional change of conductivity correspondin to a given intensity of illumination shall as great as possible. v Another object of my invention is to produce a light sensitive electrical resistance, the operatlng characteristics of which do not change with timeor use. y v A further object of my invention is to provide a method forproducing selenium relisistors which simple and mexpensiveto carry out.

Further objects of my invention will become apparent upon reading the following pecification taken in connection with the Figure 1 is a lon ltudinal section of a my invention, and p Fig. 2is a detailed view in section of a glass plate supporting a selenium conductor made in accordance with the principles of my invention. Conductors of selenium, the electrical conductivity of which varies in responseto the intensity 'of radiation incident upon them, have been previously emplo ed- 'to ermit the flow of currents in electrical circuits to be varied in accordance with an illumination to be measured. However, it has been customary heretofore to form suchselenium resistors of appreciable thickness. Selenium is a substance which absorbs light to a. considerable degree so that, as a matter of fact,

onlythe layers relatively close tothe surface of'such selenium conductors are reallyaffected by the illumination fallin upon them, thelight never reachingthe un erlyingportions of the metal. Selenium is a material having a high electrical resstivit and accordinglythe underlying layers o lthe selenium acted sim ly as dead resistance in the `electrical circult, thereby reducing the net fractional change o-f electric-resistivit for a given intensity of incident radiation. ince it isfin practically every case the fractional.

- face of a layer of metal so thin as to be sub- With the foregoing principles and objects in mind my invention is better understood nium layer to incident radiation and atthe same time to make electrical contact to an entire surface thereof, l I form one electrical contact of a highly reflecting metallic sur` face with Whichthe thin layer of seleniumis in contact, and form the opposed contact surstantially transparent to the incident radiation. By this means, radiation falling on the last named layer penetrates through it into the underlying selenium where approximate' ly half ofit may be absorbed in its first.u passage through the latter. The portion of the radiation which is not absorbed in its first passage through the selenium is reflected by the mirror surface above described, and follows a return path into the selenium, where most ofthe remaining light is absorbed. In consequence, the greatest possible fractional change ofvresistivity of the selenium is produced by this double assage of the radiation, and substantially alg the selenium present undergoes this large change in resistivity. Accordingly, the maximum possible fractional change of resistivity, for a given inci-v dent radiation, is obtained.

by reference to lFig. A'1 of the drawings in .which the reference numeral 1 designates a glass container of conventional form. Upon the interior wall of this 'container is -depos- 95 ited a layer 2 of metal, which may be gold, and which is so thin as to-transmit a substantial fraction of the radiation which may fall upon' it. Numerous methods for forming such metallic films are -well known in 100 CVI.

the art; for example, the deposit may be made by providing a filament of gold capable of being heated electrically, in the interior of the container 1. The latter may be evacuated and if the gold wire is electrically heated to a sufficient temperature, the gold will volatilize and gradually deposit as a film on the walls of the container 1. B yconling the duration of the discharge so that it transmits from fifty to eighty percent of the illumination' falling upon it.

The metallic film having been formed on the container wall as above described, a thin film 3 of selenium is deposited in turn on the surface of the gold. The selenium film may likewise be produced by cathode sputtering, .producing a discharge from a selenium cathode in a similar manner to that already described in reference to a gold cathode. The thickness of the selenium layer thus produced should be suflicient to absorb, in a single passage through it, from fifty to seventy percent of radiation incident upon it. This thickness will, of course, vary with the wave lengths of the radiation, which it is desired to absorb but vsince the (zo-efficients of absorption of-selenium'for various wave lengths are well known, it is possible to regulate the thickness to the required value without difliculty.

Alternatively, the selenium may be deposited by heating selenium in an evacuated enclosure communicating with the container 1. Selenium will vaporize and deposit on the walls of container 1 to form the desired film 3. l e

On the inner surface of the selenium layer 3 is deposited a third layer 4 of a metal having a high reflectivity for radiation of the wave lengths which it is desired to meas-- ure. For ordinary light, silver will be found a suitable metal. The metallic film L may likewise be deposited by the method of cath# ode sputtering, -or by electrically'heating a wire of the corresponding metal in the interior of the evacuated container. The layer 4 thus forms a reflecting mirror contiguous to the free face of the selenium film. The deposit of the metal 4 should preferably be so thick as to be opaque to radiation.

It' will be recognized that it is not abso-y lutely essential that4 the backing. mirror should actually be formed in contact with the selenium but that for many purposes it will be sufiicient to form the backing mirror as a separate unit; for instance, as a silvered4 glass blank, of such form that it may be positioned in contact with the selenium layer.

As an alternative arrangement, the mirror surface may be formed first, the selenium deposited upon it by evaporation of selenium through heat in vacuum, and the transparent thin gold contact layer be formed on the outside of the selenium by the evap orization methods already described.

In order to make electrical contact to the structure described in connection with Fi 1, inleading wires passing through the wa s of container l, may be provided with resillient extensions 6 and 7 bent into the form of closed rings of less diameter than the interior of lthe container 1. The wires 6 and 7 may be held as closed loops under stress by fusible links of some low melting metal while the' films 2, 3 and 4 are in course of deposition. Thereafter, currents may b'e induced in the closed loops aforesaid by means of high frethe selenium film 3 into the film 4 and out through the other wire 7.

Fig. 2 shows, in magnified detail, the above described structure of the selenium cell, the reference numeral 1 designating the container wall; 2, the contact film of gold; 3, the selenium film and, 4, the mirror backing.

While Fig. 1 illustrates a selenium film deposited in the interior of a closed -container, it will be evident that various deposits described may equally well be formed on a separate supporting member, such as a watch glass. The watch glass may be placed inside a separate containing vessel while the films above described are being deposited, the container being exhausted to a high vacuum, if necessary, in order to prevent oxidation of the films during formation.

While I have described the specific metals,

gold and silver, as being employed for the contact faces of the selenium cell, it will be understood that, for any purposes, other metals will be at least equally desirable andi that the use of such other suitable metals is within the purview of my invention. It

will also be recognized that other conductors,

which are sensitive to radiation, may be substituted for selenium without departing from the principles of my invention. q y

lSince many specific embodil'nents of the principles of my invention, other than those above described, will be evident to those skilled in the art, I desire that the following claims shall be given the broadest construction consistent with their terms and j with the limitations of the prior art. v

I claim as lmylinvention; 1. A light sensitive de vice comprising a tran arent supporting medium, a metallic film t ereon so thin as to transmit a substanthereon, v

tial fraction of radiation incident a translucent iilm on said metallic film combeam prising a conductor the resistivity of which by, and means for reiiecting the remaining portion ofsaid radiation through said film.

' In testimony whereof, I have hereunto subscribed my'name this 31st. day of October,

192s. E ZWORYKIN.,

varies in responseto the intensity of radiationincident'thereon, and a reiiecting conductor in contact with 'said translu nt film.

2. In combination with a good reiecting ness as to absorb substantially fifty percent of radiation incident thereon and a conducting lm on the free surface of said selenium so thin as to transmit a substantial fraction of radiation incident thereon.

3. A light-sensitive device comprisin a transparent, supporting medium ameta 'c iilm thereon so thin as to transmit a subsurface, a layer of selenium ofsuch thick- I st'antialI fraction of radiation. incident there-- on, a translucent Iilm on said metallic lm comprising' a conductor the resistivityof which variesin response to the variation in` the intensity of radiation incident thereon,- and -means fork reiecting the light which Passes through .said translucentf film back` therethrough. g A

4. A lig t-sensitive device comprising a transparent supporting medium, a metallic lm thereon so thnas to transmit a substantal raction'of radiation incident thereon', Va translucent lm of selenium on said metallic film, and means for passinga given h' said film ofselenium. 5. -lightsensitivebdevice comprising a transparent supporting medium, a metallic of radiant energy a plurality of times n iilm thereon so thin-ipsto transmit a substantial fraction of radiation incident there- I* on, fa translucent film on said metallic l'm comprising conducting material the resistivity of whichmvaries in response to the variation in the intensi ofradiation incident thereon, and means' or pa sing a given beam 'of radiant ener .Y plurality of times through said trans ucent film.

6. A- light#sensitive device `comprising a transparent supporting medium, a lm en said supportin medium com prisin a conducting materlal vthe vresistivity o fwhich varies intres onse to variations in the intensity ofra 'ation incident thereomand a? light-ieliecting conductor `in `vcontact with sald e 7. A- light-sensitive device comprising a transparent supporting medium, a iilm on 'said supportin medi-um comprising a con- 

